Specificity of constitutive and induced resistance: pigment glands influence mites and caterpillars on cotton plants

Cotton plants contain suites of phytochemicals thought to be important in d efense against herbivores, some of which are localized in pigment glands wh ich contain gossypol and other terpenoid aldehydes. The simple genetic basi s for the expression of these glands has led to the development of near-iso genic glanded and glandless genotypes. Glands may also be phenotypically in duced by herbivory. We determined the consequences of constitutive and indu ced gland expression on two types of herbivores, spider mites (cell content feeders) and noctuid caterpillars (leaf chewers). Induction of glands was strongly dependent on the density of attackers. Spi der mite herbivory on cotyledons (1) increased the density (but not total n umber) of glands on cotyledons linearly, (2) increased the density and tota l number of glands on the first true leaf linearly, and (3) affected the de nsity and total number of glands on the second true leaf non-linearly, comp ared to controls. Neither constitutive nor induced expression of glands aff ected mite population growth. An equal reduction of mite population size on induced glanded and glandless plants (50%) relative to uninduced controls indicated that factors other than glands were associated with induced resis tance to mites. Constitutive gland expression had a strong negative impact on caterpillar performance, reducing growth by 45%. Induced resistance to c aterpillars was three times stronger in glanded genotypes than in glandless genotypes, indicating that factors associated with induced resistance to c aterpillars are strongly associated with glands. Three cotton varieties wer e highly variable in their constitutive and induced resistance to mites and caterpillars. Thus, defense of cotton plants against herbivores can be roughly categorize d as constitutive and inducible factors associated with terpenoid aldehyde containing pigment glands that are effective against caterpillars, and fact ors not associated with glands that are effective against mites.